1. Field of the Invention
The present invention is directed to a radio-frequency antenna for a magnetic resonance apparatus, particularly for a diagnostic magnetic resonance apparatus.
2. Description of the Prior Art
For example, U.S. Pat. No. 4,825,163 discloses a radio-frequency antenna for a diagnostic magnetic resonance apparatus. The circularly polarizing antenna disclosed therein is composed of two sub-systems arranged on a cylindrical generated surface and turned by 90xc2x0 relative to one another. Each sub-system contains two antenna longitudinal conductors that are symmetrically arranged and axially directed with respect to the cylindrical generated surface. Neighboring antenna longitudinal conductors are connected to one another at their neighboring ends via resonance capacitors. Annular conductor loops (end rings) are arranged within the cylindrical generated surface and are capacitively coupled to the two ends of the two ends of the antenna longitudinal conductors. Each sub-system has a signal terminal that is connected to one of the end rings and one end of one of the antenna longitudinal conductors belonging to the sub-system.
A somewhat differently constructed circularly polarizing radio-frequency antenna is disclosed in U.S. Pat. No. 4,506,224. Similar to the antenna described above, four antenna longitudinal conductors are symmetrically arranged therein in the axial direction on a cylindrical generated surface. A radio-frequency shield surrounds the antenna longitudinal conductors. The ends of the antenna longitudinal conductors are connected via resonance capacitors to the radio-frequency shield, that additionally functions as a return conductor for the antenna longitudinal conductors.
The magnitude of the antenna impedance generally is approximately 400 xcexa9 and thus differs from the characteristic impedance of a signal line to be connected that, for example, amounts to 50 xcexa9. Without impedance matching of the antenna to the signal line to be connected, undesired signal reflections would arise at the signal terminals. Signal matching circuits that are connected between the signal terminals and the signal lines are therefore utilized. The matching circuits transform the antenna impedance to the value of the characteristic impedance of the signal line to be connected. Changes in the impedance of the antenna due to different loads as a result of subjects under examination also can be compensated by using adjustable matching circuits. A reduction of the outlay for matching the antenna to the characteristic impedance of the signal line to be connected would be desirable.
It is stated on page 452 in the book by Karl Rothammel entitled xe2x80x9cAntennenbuchxe2x80x9d, 1984, 8th revised edition, Telekosmos-Verlag, Franckh""sche Verlagshandlung, Stuttgart, that all impedances from the highest value to the zero point occur given a shorted quarter-wave line (xcex/4 line). This is utilized for the signal terminal to a dipole antenna with co-linear half-wave sections by finding the points on the conductor surface of the quarter-wave line that have an impedance corresponding to the characteristic impedance of the feed line (signal line).
European Application 0 498 571 discloses a radio-frequency antenna for a magnetic resonance apparatus that has a high-pass coil arrangement. The high-pass coil arrangement is surrounded by a radio-frequency shield. The coil arrangement has linear, conductive elements that extend in the axial direction and that are arranged with identical spacing from one another. The ends of the linear elements are connected to one another via circular conductors. This overall structure is also known as a birdcage resonator. Capacitances are inserted into the segments of the circular conductor for tuning.
An object of the present invention is to provide a radio-frequency antenna for a magnetic resonance apparatus having a signal terminal at which a signal cable can be connected with low-reflection.
This object is achieved in accordance with the invention by a radio-frequency antenna having two antenna longitudinal conductors arranged lying opposite one another and two conductor loops that are shorted in terms of radio-frequency that are respectively connected to ends of the antenna longitudinal conductors at the same side. An antenna return conductor is electromagnetically coupled to the antenna longitudinal conductors and the conductor loops. A first signal terminal location connected to the antenna return conductor and a second signal terminal location connected to one of the conductor loops are arranged such that an antenna impedance measurable over the signal terminal locations is equal to a characteristic impedance of a signal line connectable thereto. Given such an antenna structure, the current and voltage distribution which is present at the end rings is utilized to achieve the matching of the antenna impedance to the characteristic impedance of a connectable signal line without an additional matching circuit, or at most with a matching circuit having a reduced components outlay. The current and voltage distribution at the end rings occurs sinusoidally, with the current and voltage maxima being offset by 90xc2x0 relative to one another in the ideal case. As a result, four locations exist at the end rings at which voltage and current are in such a relationship in terms of the magnitude of the impedance this magnitude, for example, being 50. When one of these locations is selected as second signal terminal location, then additional matching elements can be omitted.
The return conductor may be a curved, planar conductor, such as a conductive foil.
Overall, the offset second terminal location offers the advantage that the matching of the radio-frequency antenna can be set with a reduced components outlay in any case. At the same time, the voltage load on the employed component parts such as, for example, the lines, capacitors, coils and plugs is reduced.